Proof-of-activity (PoA)

Proof of activity is a hybrid between proof of stake and proof of work. Blocks are generated through PoW mining mechanisms and then a switch to a proof of stake style mechanism occurs where validator nodes stake tokens in order to be chosen to sign these mined blocks.

Proof of activity is a consensus mechanism used in decentralized blockchain networks to validate transactions. It is a hybrid approach that combines aspects of proof of work and proof of stake consensus mechanisms.

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Proof of work and proof of stake are both designed to prevent 51% attacks - a malicious action in which one individual or party controls a majority of the mining power in the cryptocurrency network and can then include invalid transactions in the blockchain. Proof of activity combines these mechanisms into a single two step-process:

​Proof of work for miners to compete to find the block and earn the block reward. The greater percentage of the network's total computing power a miner has, the greater the probability that they will find a block.​

​Proof of stake to validate/sign the new block. The greater the percentage of the total cryptocurrency supply a staker has, the greater the probability that they will be chosen to validate the block.​

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Proof of activity adds a second line of defense against 51% attacks because an attacker would theoretically need to have both 51% or more of the network's total mining power and 51% or more of the coins staked in the network in order to successfully pull off the attack.

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​Decred is a cryptocurrency that uses the proof of activity consensus mechanism. Espers is another.